Here's some light reading on biasing. http://www.aikenamps.com/Biasing.html
The volume controller would be an attenuator. Tube amps need to be cranked up to get the best results from them. There was some discussion here http://www.guitartricks.com/forum/showthread.php?t=28216&page=2&pp=7 about attenuators, where this link was provided as an example http://www.humbuckermusic.com/drzairbrliby.html
You could probably build one much cheaper if you were electronically inclined. :)
...so ever since then, I always hang on to the buckle.